Radio communication systems are known to include a plurality of communication sites that facilitate communications amongst a plurality of communication units. A collection of such communication sites is commonly referred to as a zone, (e.g., SmartZone.TM. Systems as manufactured by Motorola, Inc.). Generally, zones can be characterized as a plurality of overlapping coverage areas resulting from spatially separated communication sites. Communication units within the overlapping coverage areas route their calls through any one of a plurality of usable communication sites (i.e., those through which control and audio signaling can be readily exchanged). In an effort to expand the coverage areas of communication systems, system installers are beginning to link the zone communication systems together, e.g., the SmartZone OmniLink.TM. System, as manufactured by Motorola, Inc. Linking communication zones in this manner, however, creates a number of resource allocation problems not present in single zone systems. Principal among these problems is the undesirable use of interzone audio resources, as next described.
By way of example, FIG. 1 shows a multizone communication system 100 that includes zones 102, 103 whose sites are linked to zone controllers 105, 106 respectively. Audio signals are routed between the two zones, 102, 103 via audio switches 107, 108, by way of interzone audio resources 109. These interzone audio resources 109 might be comprised of leased DS1 circuits, analog or digital circuits, or private microwave facilities.
Communication units 111-117, which may be mobile radios, portable radios, dispatch consoles or the like, are distributed about the multizone communication system 100. When establishing communications amongst the many communication units, there are three possible audio paths that might be required. The three audio paths are: 1) intrasite; 2) intersite in the same zone; and 3) intersite across two or more zones. The first path, intrasite, refers to calls between members of a talkgroup that are all located within the coverage area of a single site (e.g., mobiles 111 and 112). The second path, intersite in the same zone, refers to calls between some talkgroup members that are within the coverage area of a first site, and others that are within the coverage area of a second site within the same zone (e.g., mobile 111 and portable 113). The third path, intersite across two zones, refers to calls between members of the talkgroup that are within the coverage area of a first site in a first zone, and other members of the talkgroup that are within the coverage area of a second site within a second zone (e.g., portable radios 116, 117). Of these scenarios, the first two create no special hardships for the subscriber or the system. That is, they don't result in any extraordinary access time delays or undue channel congestion, as no interzone audio resources are required to facilitate such communications.
By contrast, the third scenario requires not only the usurpation of scarce interzone audio resources 109, but additionally, there are at least two access related problems created by utilizing this audio path. In particular, there may be on the order of a 200-300 millisecond delay in the call setup, thereby resulting in audio holes and delayed communications. More critically, the probability of being granted one of these scarce audio resources 109 is dramatically decreased as the system becomes more congested. While the time delay problem is limited by switching or processing speed, the access probability can only be improved by adding more interzone audio resources 109. Unfortunately, communication resources of this type are rather costly, which drives up the system operating costs dramatically. Accordingly, a need exists for reducing the dependency on expensive audio resources between zones, if possible, without impacting the ability of communication units to communicate with each other.